1. Field of the Invention
The invention relates to a fuel supply system, especially for a turbojet aero-engine, and is particularly concerned with fuel cooling means in such a system.
2. Summary of the Prior Art
Fuel supply systems in aircraft turboe-engines generally comprise a low pressure pump, usually of the centrifugal type, which receives fuel from a booster pump disposed in the fuel tank of the aircraft, and a high pressure displacement pump which receives fuel from the low pressure pump. On leaving the high pressure pump the fuel is delivered through a feed flow regulator towards an injection system.
A control valve in a branch at the outlet of the high pressure pump permits a portion of the fuel to be diverted and recycled upstream of the high pressure pump under the action of a control pressure in such a manner that the pressure drop across the feed flow regulator remains constant.
The fuel which flows in this type of supply system may be subjected to substantial temperature variations. For example, on issuing from the low pressure pump, the fuel may be at a temperature T1 of 20.degree. C. or so, whereas after it has passed through the high pressure pump, and therefore into the control valve branch, the same fuel can have undergone heating which raises it to a temperature T2 close to 100.degree. C., or even higher.
Furthermore, under certain flight conditions, at high altitude and low speed for example, the flow injected into the combustion chamber of the engine by the injection system is low, whereas the high pressure pump, rotating at high speed, has a high delivery rate. The portion of the flow diverted by the control valve is thus substantial, and the diverted fuel, at temperature T2, is reintroduced into the system upstream of the high pressure pump. The fuel which enters the high pressure pump is therefore a mixture of a relatively low proportion of "cold" fuel coming from the low pressure pump and a high proportion of recycled "hot" fuel coming from the control valve, and it is at a temperature T close to T2. As a result of this high temperature T at the inlet of the high pressure pump, the fuel is heated up still further and brings about a new high temperature T2. Such overheating of the fuel may bring about its decomposition (if the temperature T2 exceeds 150.degree. C.) and cavitation phenomena in the high pressure pump.
In addition, if the fuel of the supply system is used as a cooling agent in a heat exchanger to cool, for example, oil circuits associated with the engine, excessive heating up of the fuel causes the latter to lose its cooling power. Such a heat exchanger is generally disposed in the fuel recycling circuit, downstream of the control valve or upstream of the high pressure pump, where conditions are the most favourable for its operation, for although the fuel there is hotter than at the outlet of the low pressure pump, the rate of flow there is much greater under the most critical conditions, particularly when flying at high altitude and low speed. However, with such an arrangement it is necessary in some critical cases to be able to reduce the temperature T2 if it is too high for the exchanger to play its part properly.
This problem could be solved by installing an air-fuel heat exchanger specifically to cool the fuel in the particular flight stages referred to above. However, the overall size and weight of such an exchanger, useful only in these particular flight stages, would penalize the performance of the engine and the aircraft in all other flight regimes.